1. Field of the Invention
The present invention generally relates to bis(benzoxazole) compositions and methods of making the compositions. More particularly, the invention relates to analogues of natural occurring bis(benzoxazole) compositions and methods of making bis(benzoxazole) analogues.
2. Description of the Relevant Art The relatively simple yet novel bis(benzoxazole) natural products UK-1 and AJ19561 represent a new class of cytotoxic secondary metabolites. Interest in this class of bis(benzoxazoles) is fueled by the finding that UK-1 is selectively cytotoxic to cancer cells but not bacterial cells, yeast, or fungi. This selective cytotoxicity may be mediated through the specific interaction of UK-1 with an as yet undefined cancer cell-associated target. Bis(benzoxazole) natural products are a structurally unique class of Streptomyces secondary metabolites that have recently been reported in the literature. Research groups have isolated UK-1 from acetone extracts of Streptomyces sp. 517-02.
Research groups have isolated AJI9561 from Streptomyces sp. AJ956.
UK-1 and AJI9561 were reported to possess growth inhibitory properties against murine cancer cell line P388, with IC50 values from 0.3-1.6 μM. While UK-1 has cancer cell cytotoxic properties, UK-1 does not inhibit the growth of gram-positive or gram negative bacteria, yeast, or fungi at concentrations as large as 250 μM.
The selective cytotoxicity of UK-1 towards cancer cells versus bacteria and fungi indicates that UK-1 may have a unique mechanism of anticancer action. The 2-(2′-hydroxyphenyl) benzoxazole moiety present in UK-1 is also present in a number of synthetic metal ion chelators and is analogous to the 2-(2′-hydroxyphenyl)oxazole moiety present in a class of microbial siderophores.
The cytotoxicity of UK-1 against P388, B 176, and HeLa cell lines have been previously reported.
Cytotoxicity of UK-1 against Cancer Cell Lines.
Cell LineIC50aMCF-7 1.6 μMHT-29  65 μMHL600.32 μMPC-3 0.4 μMMDA-231 0.5 μMBT-200.17 μMDU145 0.2 μMSKBR3 0.1 μMA549 1.9 μMSK-N-AS  24 μMSK-N-D70.17 μMSK-N-F1  7 μMSK-N-MC 1.1 μMSK-N-SH  7 μMCHP-212  12 μMIMR-320.06 μMNGP0.02 μMadetermined using the alamarBlue assay. See Examples for details.UK-1 displays a wide spectrum of potent anticancer activity against leukemia, lymphoma, and certain solid tumor-derived cell lines. In particular, certain neuroblastoma cell lines (IMR-32 and NGP) were very sensitive to UK-1, with IC50 values less than about 100 nM. Other cell lines, such as colon (HT-29), were less sensitive. Despite the potential anticancer activity, there was no indication of any antibacterial effect against Staphylcoccus aureus, methicillin-resistant S. aureus, or Pseudomonas aeruginosa at concentrations up to 50 μg/mL of UK-1.
The metal ion binding ability of synthetically prepared UK-1 has also been investigated. The studies indicate that UK-1 is capable of binding a variety of biologically important metal ions, of particular interest, Mg2+ ions. UK-1 binds DNA in a metal ion-dependent fashion like the Mg2+ binding aureolic acid group of antitumor antibiotics and synthetic antitumor quinobenzoxazines. One consequence of the interaction of UK-1 with DNA is the inhibition of topoisomerase II.9 A more recent investigation of the metal mediated double strand DNA (ds) binding by Electrospray Ionization Mass Spectrometry (ESI-MS) demonstrated that UK-1 forms complexes of the type [ds+UK-1+M2+] with a variety of metals, such as Ni2+, Co2+, and Zn2+. The results distinguish the metal mediated DNA binding of UK-1 with the observed metal mediated DNA binding of aureolic acids and quinobenzoxazines. Aureolic acids form metal mediated DNA binding complexes of the form [ds+2× ligand+M2+]. Quinobenzoxazines bind as [ds+2× ligand+2×M2+] metal mediated DNA binding complexes.
The semi-synthetic derivatives methyl UK-1 (MUK-1) and demethyl UK-1 (DMUK-1) both have shown activity against Gram-positive and Gram-negative bacteria. MUK-1 also is active against yeast and filamentous fungi.
